Change of deformation mode caused by aging of Ti-14 mass%Mo alloy single crystals and embrittlement mechanism due to the ω-phase

A change of deformation modes in lattice instable Ti-14 mass%Mo alloy single crystals upon aging treatments at 623 K was investigated in the tensile test up to rupture. The tensile direction of these specimens was [116]_B. The main deformation modes of as-quenched specimen (AQ) were {332} 〈113〉 and {112} 〈111〉 mechanical twins. Their Schmid factors (SF) were 0.321 and 0.156 in {332} 〈113〉 twins and 0.447 in {112} 〈111〉 twins. A specimen aged for a short time of nearly 250 s (AG (S)) showed a kind of transitional state of deformation mode which involved {332} 〈113〉 twins, {112} 〈111〉 twins and {112} 〈111〉 slips. In this state AG(S) had been already poor in ductility. A specimen long aged for 1 × 10⁵ s (AG(L)) showed a remarkable brittle fracture caused by the aged ω phase, but it was not the {001} cleavage fracture. The fracture surface had a chevron pattern induced by the operation of some {112} slip systems in the initial stage. Large grown aged ω variants caused the B matrix to remarkably increase the stress level for deformation and induced locally an Ω→B inverse transformation. Local softening which occurred with the inverse transformation led to an extreme stress concentration and then to crack. In this way AG(L) apparently showed notable embrittlement.